PONs are used in point to multi-point communications applications. FIG. 1 illustrates a simple example of a PON 10. An optical line terminal (OLT) 12 is connected to the “head end” of the PON and is typically located in a local telephone exchange (a central office). The OLT 12 controls access to the shared PON and interconnects the PON with a wider telecommunications network. Examples of outside services connected to the PON may be cable television (CATV) 14, an Internet network 16 (for VoIP and data), and any other wide area network (WAN) 18. A connector bus or switch 19 connects the signals from the various services to the OLT 12 ports. The OLT 12 communicates with the bus 19 using serial or parallel electrical signals in well known formats.
The OLT 12 manages the incoming data from the outside sources, converts the data to light pulses, and transmits the data via one or more fiber optic cables to a plurality of optical network units (ONUs) 20, 21, 22, which are the user ends of the PON, typically up to 10 km downstream from the OLT 12. The ONUs are connected via wires to the ultimate users 23-25. The OLT 12 also manages optical transmissions from the ONUs 20-22 to the outside network. If the fiber were run all the way to a home or office building an Optical Network Termination (ONT) would be needed. Whether the termination is an ONU or an ONT is not relevant to the present invention.
A PON is very efficient since only passive splitters 28 are used in the fiber optic network. The splitters 28 couple the fiber optic cable 34 from the OLT 12 to each fiber optic cable 30-32 leading to an ONU 20-22. In a PON system, a light signal from a single fiber optic cable may be split into 64 or more fibers.
Standards for PON are described in various publications, such as ITU-T-G.984 (Gigabit PON). All these applicable standards are well-known to those skilled in the art and are incorporated by reference.
A transmitter 36 in the OLT 12 converts electrical data to light pulses using a laser diode. Light is transmitted by the OLT 12 to the ONUs at one wavelength, and light is transmitted by the ONUs back to the OLT at a different wavelength, so there is wavelength division multiplexing (WDM) in the PON.
A receiver 38 in the PON converts the optical signals received from the ONUs to electrical signals.
A media access controller (MAC) 40 controls the communications over the PON and the formatting of the data (e.g., packetizing, depacketizing, serial-parallel conversion, etc.). Data passing “upstream” over the PON from the ONUs to the OLT 12 are typically multiplexed according to a Time Division Multiple Access (TDMA) technique in which data channels are separated in time, using assigned time slots, to avoid collisions at the OLT 12. The OLT 12 transmits the data from the outside networks to the ONUs typically using a broadcast scheme, and the particular ONU having the destination address specified in the transmission then processes the data. The non-addressed ONUs ignore the transmission. Encryption is used for security.
The data coming from the ONUs is transmitted in packets using a certain protocol standard. Various protocols, known as Media Access Control (MAC) protocols, have been developed to control an ONU's upstream access to the shared capacity on a PON. MAC protocols may implement the TDMA multiplexing scheme in the upstream direction, or other packet-based data transfer schemes may be used that are more appropriate to especially high data rates or to a variable rate asymmetric data transport.
A typical PON configuration does not permit ONUs to communicate directly with each other and requires the MAC 40 to determine the order of transmissions and the time of transmission.
One popular type of MAC protocol described in the ITU standard for GPON specifies a minimum 32 bit guard time between packet cells to prevent collisions, a 44 bit preamble of alternating 1s and 0s for bit synchronization, a 20 bit delimiter to indicate the start of incoming payload data, followed by the fixed or variable length payload data. The payload data includes addresses and the primary data information. A simplified version of this protocol is illustrated in FIG. 2.
Since each ONU 20-22 is at a different distance from the OLT 12, the round trip time for a packet will be different for each ONU. The MAC 40 in the OLT 12 has a stable reference clock that is used for the processing of the incoming digital signals. Since it is important that the bits from all the ONUs be received by the OLT 12 in phase, the MAC 40 introduces a phase correction for each ONU to use when transmitting so that all the ONUs have the same constant equalized round trip delay. This is called ranging.
The MAC 40 in a GPON system issues a programmable reset signal shortly after the end of a packet burst to reset the protocol sequence and any other circuitry needing a reset. The reset pulse ends shortly before the preamble. The reset pulse occurs during the guard time between bursts of data. Such MACs are well known and commercially available.
With data rates of 1.25 and 2.5 gigabits per second, and with the magnitudes of the light pulses from each ONU being different, conversion of the pulses of light to error-free electrical digital signals is very difficult. In a PON receiver, a photodetector converts the magnitude of a light pulse to a proportional analog current. This current is converted into an analog voltage by a transimpedance amplifier (TIA), and the output of the transimpedance amplifier is applied to a limiting amplifier (such as a comparator) that determines whether the analog signal is a logical 1 or a logical 0 bit. (The term “analog” is used herein even though the data transmitted is digital because the amplitudes of the logical 1 and 0 bits are variable due to the different distances of the transmitters.) The limiting amplifier then outputs a clean and valid digital signal.
The threshold voltage of the analog signal that the limiting amplifier uses for determining whether the light pulse is a logical 1 or a logical 0 is difficult to quickly establish since the magnitude of the light pulses received by the OLT vary for each ONU. The threshold voltage is optimally the midpoint between the voltage amplitudes of a logical 1 and logical 0.
For example, FIG. 2 illustrates two simplified analog signals 44 and 46 outputted by the transimpedance amplifier for a “close” ONU 20 and for a “distant” ONU 22, respectively. The optimum threshold voltage level 48 for determining whether the signal is a logical 1 or a logical 0 is ideally the midpoint between the peak voltage and minimum voltage. At very high speeds, it is very difficult to quickly establish the threshold voltage at the midpoint, as this is often implemented using two peak detectors and a resistor divider to detect the minimum and peak values. Not setting the threshold at the midpoint increases the chances of bit errors.
In another possible technique, the threshold voltage for determining whether the analog signal is a logical 1 or a logical 0 may be derived by obtaining the average magnitude of the analog pulses over time. The average may be obtained using a low pass filter (e.g., a capacitor and resistor having an RC time constant) to extract the DC component (assumed to be the average) of the data stream. If the analog signal is above the average of the data stream, it is assumed to be a logical 1. However, to prevent a series of 1s or 0s from significantly varying the threshold voltage, the time constant of the low pass filter must be relatively long/slow. A long RC time constant would result in a relatively long time, starting at the beginning of a packet cell, to establish an average since the filter capacitor voltage begins at an arbitrary voltage resulting from a previous burst from a different ONU. This would result in a high error rate until the capacitor voltage stabilized.
What is needed is an improved technique for determining whether an analog signal in a high data rate PON system, or other digital burst-mode system, is a logical 1 or a logical 0.